Sidedness sounds like a detail; in pelvic vein disease it's anatomy speaking. The venous system isn't symmetric: the left ovarian vein drains at an awkward right angle (the right one joins the main body vein more smoothly), and two named compression problems — May-Thurner and Nutcracker — both squeeze left-sided veins. So pain that consistently favours one side, especially the left, raises specific and findable causes.
A clear one-sided answer also does quiet work at appointments: "left-sided, worse standing, since my second pregnancy" is a pattern a vein specialist recognises immediately, where "pelvic pain" alone starts the long generic work-up.
Right side or both sides? Still meaningful — right-sided reflux exists, and both-sided patterns are common; the FAQ covers exactly this. And pain with no consistent side simply moves the weight onto your other answers. There are no wrong answers here — only information.